Amniotic Fluid Dynamics - Significance

Introduction

Amniotic fluid is an odorless, transparent, or yellow-colored fluid inside the uterus, surrounding the fetus.

When fetal development occurs inside the amniotic sac, the amniotic fluid acts as a cushion between the umbilical cord, and the fetus, thereby reducing the compression risk between the uterine wall and the fetus. It also acts as a reservoir for the fetus as it contains essential antibodies, nutrients, and hormones from the mother. It provides essential growth factors and fluid for the growth and development of fetal organs and the fetus. The amniotic fluid also physically protects the fetus in case any abdominal trauma occurs to the mother. It also has antibacterial properties that protect the fetus from infections.

A developing fetus obtains large volumes of water through maternal circulation from the placenta. During the second half of gestation, amniotic fluid is mainly obtained from the lung fluid and fetal urine. The fluid is then resorbed by fetal swallowing.

What Is Amniotic Fluid Dynamics?

Amniotic fluid predominantly consists of water (98% to 99 %). An average amniotic fluid volume is essential for the growth and development of the fetus. The amniotic fluid mirrors or reflects the metabolic status of the fetoplacental unit. The amniotic fluid volume thus consists of water flowing in and out across the amnion and placental membrane. The amnion limits the intramembranous water flow, and in the placenta, it is limited by the syncytiotrophoblast. In humans, the placental permeability increases with gestational age. Therefore, with gestational age, the chemical composition of its contents also varies. An amniotic fluid volume equilibrium is maintained by resorption (intramembranous flow and swallowing) and fetal fluid production (urine and lung liquid).

Regulatory (inflow and outflow) mechanisms that keep the amniotic fluid volume within an adequate range mainly act at three levels:

1. Transfer of the solute and placental control of water.

2. Regulations of fetal outflows and inflows.

3. Maternal effect on the fluid balance of the fetus.

An amniotic fluid dynamic consists of its production phase and elimination phase.

1. Amniotic Fluid Production:

• First Half of Pregnancy - During the first half of pregnancy, the amniotic fluid is obtained from the maternal and fetal compartments. This water arrives across the placenta from the mother's circulation along with solutes and travels through the fetus's skin, chorion, and amnion. It also circulates between amniotic fluid, the placenta, the fetal body, and fetal water compartments. The amniotic fluid present in the early gestation stage is similar to that of maternal and fetal plasma but with low protein concentrations. Until 14 weeks of pregnancy, neither swallowing nor fetal urination contributes to the amniotic fluid volume.

• Second Half of Pregnancy - During the second half of pregnancy, when the fetal urine starts entering the amniotic sac, the osmolarity of the amniotic fluid decreases. It decreases further with the advancement of gestational age and keratinization of the fetus's skin. Keratinization of the fetus's skin begins at around 19 to 20 weeks of gestation and by 25 weeks. The low amniotic fluid osmolarity is a potential force for an outward water flow across the transmembranous and intramembranous pathways. The intramembranous pathway transfers the solutes and fluid from the amniotic cavity to the fetus's circulation across the amniotic membrane. The transmembranous pathway involves a direct exchange across the fetal membranes within the uterus between the maternal blood and fetus. Osmotic and hydrostatic forces drive the water flow through these membranes. Therefore, any change in the osmotic and hydrostatic forces can alter the water flow between the mother and the fetus.

2. Amniotic Fluid Elimination: Fluid resorption occurs through fetal swallowing, respiratory tract, placental transfer, and an intramembranous pathway (from the amnion towards the fetus's circulation).

• Amniotic Fluid Removal Through Fetal Swallowing: During fetal swallowing, the fluid is absorbed through the gastrointestinal system of the fetus, which is either moved to the maternal compartment along the placenta or filtered by the kidneys.

• Amniotic Fluid Removal Through the Respiratory Tract: Since the amniotic fluid is more hypotonic than the fetal plasma, exposure of the amniotic fluid to the fetal alveolar capillary bed results in water movements from the amniotic cavity to the fetus. Amniotic fluids are also removed by a continuous bulk flow caused by the oncotic and hydrostatic forces. Transport across the amnions is modulated by the prolactin levels of amniotic fluid and occurs through the intercellular channels between the amniotic epithelial cells.

• Amniotic Fluid Removal Through the Placenta: The large section of the intervillous or capillary interface of the fetus magnifies the osmolar gradient between the fetus and the mother, thereby transferring vast volumes of water. The exchange of water at this level influences the intravascular volume of the fetus and possibly affects the production of urine and renal blood flow.

A bulk flow of liquid occurs through phasic pathways, like swallowing and micturition, and nonphasic, which is usually mediated by oncotic and hydrostatic gradients and through bidirectional water flow between the maternal and amniotic compartments.

Does Amniotic Fluid Dynamic Affect Maternal and Fetal Health?

Abnormal amniotic fluid volumes cause maternal disease, fetal disease, or both. An appropriate amniotic fluid dynamics is essential, as excessive (polyhydramnios) or insufficient (oligohydramnios) amounts of amniotic fluid are associated with impaired or poor fetal outcomes. Polyhydramnios occurs alongside conditions like gestational diabetes, multiple pregnancies (triplets or twins), and congenital anomalies. Oligohydramnios occurs with conditions like placental dysfunction, fetal abnormalities; ruptured membranes, and late pregnancies.

How Is Amniotic Fluid Assessed?

The amniotic fluid index is used to measure amniotic fluid sufficiency in pregnancy. A standard amniotic fluid index varies from 5 cm to 25 cm. The amniotic fluid index of polyhydramnios is greater than 25 cm, and the amniotic fluid index of oligohydramnios is less than 5 cm.

What Is the Significance of Understanding Amniotic Fluid Dynamics?

1. As the amniotic fluid mirrors or reflects the metabolic status of the fetoplacental unit, the study of amniotic fluid components and their variations during different weeks of pregnancy provides indications for placental insufficiency, an appropriate assessment of the fetus's maturation, and evaluating the kidney function parameters.

2. Studies on regulating the amniotic fluid volume can be used to understand the mechanisms the fetus uses in maintaining water homeostasis.

3. Amniotic fluid is assessed in patients with premature rupture of membranes or preterm labor to investigate any intraamniotic infection.

4. Amniotic fluid indicators like elevated levels of matrix metalloproteinase are usually suggestive of an infection.

5. New treatments for abnormal fluid volume abnormalities are developed by appropriately understanding the mechanisms, resulting in an excellent clinical outcome.

Conclusion

The amniotic fluid is dynamic throughout the pregnancy, which is essential for the fetus's well-being. The amniotic fluid volume mainly includes the production and elimination of amniotic fluid. It also consists of water flowing in and out across the amnion and the placental membrane. Abnormal amniotic fluid volumes cause maternal disease, fetal disease, or both. Therefore, an average amniotic fluid volume is essential for the growth and development of the fetus.